Sonars and radars can explore distant targets in wide ranges. Unfortunately, they have low resolutions along the travel line (azimuth direction) of platforms with the sonars and radars in the exploration of distant targets. A synthetic aperture processing method is known for overcoming such a disadvantage in which a virtual extension of a short receiving array of a readily manageable size enhances the azimuth resolution.
In the synthetic aperture processing, while moving at a constant speed, the platform repeatedly radiates periodic pulses toward the ground or sea surface to receive a plurality of reflected waves. Subsequently, processing of these refection waves creates sonar images or radar images with improved resolutions in both the range direction (direction of the pulse-irradiated ground or sea surface) and the azimuth direction.
Known synthetic aperture processing methods are one-dimensional algorithm and two-dimensional algorithm (see, for examples, Non-patent literature 1). In the one-dimensional algorithm, the reflected waves are pulse-compressed in the time direction (referred to as range compression) based on a conventional pulse compression technique. Subsequently, the recorded time lag of the reflected waves, called range coverture, which is caused by the movement of the platform, is corrected. Waveform amplitudes of the reflected waves are then accumulated to output a sonar image or radar image with improved resolutions in both the range and azimuth directions.
In the two-dimensional algorithm, a plurality of the reflected waves are two-dimensionally correlated with reference signals that are produced based on ideal reception waves reflected from a plurality of predetermined sites within the coverage, and correlation values are calculated for these predetermined sites within the coverage. Based on the correlation values, a sonar or radar image with improved resolutions in both the range and azimuth directions is produced.    Non-patent Literature 1: Kazuo Ouchi, “Rimoto Sensingu no tameno Gousei Kaikoh Rehdah no Kiso (Basic Synthetic Aperture Radar for Remote Sensing)”, Tokyo Denki University Press, 2004